Once-through vapor generator start-up system



Aug. 29, 1967 W, p. GORZEGNO ET AL 3,338,055

ONCETHROUGH VAPOR GENERATOR START-UP SYSTEM 4 Sheets-Sheet l Original Filed May 20, 1963 NNJ Ric/70rd h'. 7' homas ATTORNEY Aug. 29, 1967 W p GORZEGNQ ET AL 3,338,055

ONCE-THROUGH VAPOR GENERATOR START-UR SYSTEM 4 Sheets-Sheet 2 Original Filed May 20, 1963 Oom- Cov

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0 S MIM EMV n# 0 8 .S J P. D n um w w Rfk/10rd H Thomas ATTORNEY Aug. 29, 1967 W p, GORZEGNO ET AL 3,338,055

ONCE-THROUGH vAPc-a GENERATOR START-UP SYSTEM 4 Sheets-Sheet 3 Original Filed May 2O, 1963 @fc/10rd H 7/7 omas Aug. 29, 1967 W p. GQRZEGNO ET AL 3,338,55

ONCE-THROUGH VAPGR GENERATOR START-UP SYSTEM Original Filed May 20, 1963 4 Sheets-Sheet 4 TURB/N6 o :o 2o 3o ao so en 7o ao 9o foo f pfecf/vr LOAD l Opf/v IDO 0 l0 20 30 10 SO 60 70 8O 90 |00 PERCENT OAD I l ma l c//ecu/rJ INVENTOR. mum e @onza-afm Ame/avr J. Z/MY WILL/AN a. STEVE/vs Richard H Thomas Afro @Iver United States Patent O 11 Claims. (Cl. 60-107) ABSTRACT F 'THE DISCLOSURE A once-through vapor generator-turbine control system in which a load signal controls fuel and air input to the unit, including means for modifying the signal relative to turbine -throttle steam temperature and temperature of the vfeed flow to the unit, the unit further including means to divert at low loads and start-up a variable amount of ow which preheats the feed ow.

This application is a division of application Ser. No. 281,452, filed May 20, 1963.

This invention relates to a start-up and low load system for vapor generators, and in particular to apparatus, methods and controls for starting-up, re-starting and lowload operation of a sub-critical or super-critical vapor generator of the forced ow once-through type.

An important item in the operation of a once-through vapor generator is the by-pass system around the turbine. This system is necessary to protect the furnace walls by providing sucient circulation of fluid at start-up and low loads. However, the quantity of water 'or steam owing through the tubes, to prevent the tubes from overheating and burning out, may exceed the -turbine capabilities, and the by-pass system then passes the excess ow around the Iturbines.

Protection of the high pressure turbine parts also is an important matter in the design of a once-through generator. These parts, which must be capable of handling flows at high temperatures and at very high pressures, for instance, 3500 p.s.i., constitute a large portion of the expense of a turbine unit. Avoiding excessive thermal stresses in the parts is, therefore, a critical consideration.

It is known in by-pass systems to use a flash drum in the turbine by-pass line for the purpose` of handling the excess ow and separating the vapor content from the iiow for warming and'rolling of the turbine. It is also known in start-up systems to use a throttling valve upstream of the turbine for the purpose of furnishing reduced pressure vapor to the turbine. This low pressure throttle vapor allows start-up operation of the turbine with minimum temperature diierentials (and therefore minimum thermal stress) in the turbine inlet parts.

The objectives in such prior systems include the avoidance of excessive thermal losses, the reduction of the time required for start-up, and the exercise of Care in the protection of turbine parts and vapor generating and superheating sections against thermal shock and high temperature damage. It is found that the application of principles in accordance with the present invention results in substantial improvements in -these respects beyond those heretofore achieved.

Accordingly, iit is yan object of the invention to provide a system in which, during start-up, or low-load operation, the heating surfaces are more fully protected, and in which steam is made available earlier in the start-up period for warming-up -and'rolling the turbine, and for other uses, to reduce to a minimum the period required 3,338,055 Patented Aug. 29, 1967 ice for start-up and, incidental to this, to reduce the heat losses which result from start-up and low load operation. It is also an object of the invention to supply the turbine with steam at a pressure and temperature which is gradually increased during the start-up period and which reaches full throttle pressure at a load more compatible with the turbine design for maximum protection of turbine parts.

These and other objects are accomplished in accordance with the invention by providing in a once-through unit,

which includes a vapor generating section and superheating sections, means upstream of the superheating sections for reducing the uid pressure so that heat is imparted to the fluid in the superheating sections at a reduced press-ure; For a given flue gas flow over a superheating bank, and for a given uid ow within the bank,

for the same thermodynamic entering conditions of flue gas'and uid, the reduction of the uid pressure greatly increases the amount of heat absorbed by the uid flowing through the bank.

This scheme provides the necessary vapor for rolling and warming the turbine earlier in the start-up cycle than if heat was imparted to the uid in the superheating sections or banks at Ifull pressure. In addition, a shorter period for start-up and rolling contributes, in conjunction with suitable heat recovery circuits,towards obtaining a minimum loss of start-up heat input.

Also provided in accordance with the invention is a by-pass system which includes a flash tank disposed between or intermediate superheating surfaces or sections arranged to receive the reduced pressure flow which has been reheated following the reduction in pressure in the upstream superheating section. Also included in the by-pass system are piping and valves for the distribution of the by-pass flow to various areas of a condensate-V a greater amount of vapor is flashed in the ash tarikearlier in the start-up cycle. Since at least a portion of the vapor is then passed to the superheating section downstream of the ash tank, this portion is supplied to the turbine in a superheated state. In this Way the turbine gets a superheated vapor earlier in the start-up period, and, in a manner to be described, a vapor ow at a heat level required for maximum protection of turbine parts. In operation of the start-up and low load system, in accordance with the invention, the method of operation includes the steps of iirst establishing the required ow and pressure for cooling the circuitry and then placing the 'burners in service at a reduced tiring rate. The iluid pressure is reduced at an intermediate point in the heating surfaces of the generator, and the circulating fluid is reheated at the reduced pressure. The Vreheated uid Vis ashed to provide a vapor flow, Whichis further heated, the amount of ow being adjusted to achieve, in the step of further heating, a desired level of heat content.

In this respect, for a cold start, the ow may be adjusted to provide to the turbine throttle, for warming and rolling the turbine, vapor in a slightly superheated state. In a hot re-start, the vapor is superheated to a level, at which, after throttling through a conventional turbine throttle by-pass valve, the temperature of the vapor matches the temperature of the turbine inlet parts.

When the turbine reaches a steady state condition, it is synchronized and loaded, and the load is increased by increasing the ow to the turbine up to full start-up ow established in the high pressure circuitry, and then by increase of the temperature, pressure, and flow through Y ,tion and accompanying drawings, in which:

FIGURE 1 illustratesV schematically an embodiment of a start-up system in accordance with the invention;

FIGURES 2, 3, and 3A are temperature-enthalpy diagrams illustrating operation of typical high temperature convection surfaces for a cold start and a hot re-start, respectively, in accordance with the invention;

FIGURE 4 illustrates a throttle valve and governor valve control arrangement for a high pressure turbine;

FIGURE 5 is a diagram showing operation of a turbine-generator unit in accordance With the invention during a cold start with respect to variation in pressure with percent load at'the turbine throttle;

FIGURE 6 is a diagram for the unit of FIG. 5 Yshowing the variation of the turbine governor valve position with load; and

FIGURE 7 is a diagram illustrating a main ow line reducing station in detail in accordance with an embodiment of the invention.

' Referring to the embodiment of FIG. l, there is illustrated schematically a vapor generating and turbine installation which includes in series an economizer 12, furnace passes 14, and roof and convection enclosure passes 16. Also constituting sections of the installation are a primary and/ or platen super-heating section 22 and a finishing superheating section 24. During normal operation of the unit, the ow is through the superheating sections and from the outlet of the finishing superheating section 24 vto a high pressure turbine 26, the exhaust steam from the turbine being reheated at 28 and passed to a low pressure turbine 30, and from there to a condenser 32. Being of the once-through type, the system is pressurized by a feed pump 34, the feed flowing from the condenser through low pressure heaters 36,` deaerator 38, storage tank 40 and high pressure heaters 42 to the economizer 12.

In accordance with the invention, the start-up system includes a pressure reducing station 44 (which may constitute a plurality of valves, although, for the purpose of illustration, only a single valve is shown) in the main ow-path upstream of the primary superheater 22 receiving flow from the roof tubes 16. Also constituting part of the start-up system is a ash tank 46 disposed in a Iby-pass line 48 leading from the outlet of the primary superheater 22 and by-passing a main ow line valve (or valves) 50 between the superheating sections 22 and 24. A stop-valve 52V in the by-pass line 48 separates the ash tank from the outlet of the primary superheating section 22. Also disposed in the 'by-pass line 48 is a spray attemperator 54, and a supplemental by-pass line 98 extending around the stop valve 52 between the primary superheater outlet and the flash tank.

The piping and valves for the distribution of the bypass ow from the ash tank consist of a line 56 extending to the inlet end of the finishing superheater 24, this line containing a valve 58; and a vapor line 60 leading from the vapor space of the iiash tank including branches 60a to theturbine gland seal regulator, 60b to the deaerator, and 60a` to the high pressure heaters, these lines be'ing opened and closed by valves .62, 64 and 66 respectively. Drain ow Vfrom the flash tank is handled by a line 68 having branches 68a and 68b leading to the condenser hot-Well 7 0 and to the high pressure heaters 42, respectively, these lines being opened and closed by valves 72 and 74, respectively. g

Also constituting part of the by-pass system is a line 76 leading from the outlet of the finishing superheater 24 to the condenser, by-passing the high pressure and low rares-V sure turbines, a pressure reducing valve 78 in the line, and

a spray attemperator 80 immediately upstream of the entrance port for the condenser. The line 76 is joined by a branch line 60d, from the ash tank vapor space through valve 82.

The invention will be better understood by way Vof example, With 'reference to FIGS. 2-6, FIGS. 2, 3 and 3a showing steps of operation of a once-through unit in accordance with the invention, and FIGS. 4-6 showing the manner of tie-in of the unit with a turbine. Although in the lfollowing example, specific numbers are given with respect to pressures, flow rates, temperatures and the like,

it is to be understood that the concepts of the invention are not limtied to thesepspeciflcs.

' critical unit, 255() being that for a subc-ritical'unit.) At Y' Initial helating of once-through unitV Initially, in a cold start-up period (FIG. 2), the feed Y pump is driven to pressurize the unit upstream of the returbine is incapable of handling this flow, the output from the superheater is fed through stop valve 52 to the ilash tank 46, and'through the liash tank drain line 68` to the condenser hot well and the second stage of the high pressure heaters. In the example, a ow rate of 30% of full load flow through the unit is required for satisfactory cooling of the high pressure circuitry. The pressure upstream of the reducing station is initially held at 600 p.s.i., and the uid is throttled through thereducing valves to a pressure of about p.s.i If the upstream pressure were held at a higher value, the high velocity dow would seriously erode the valves. The lower upstream pressure permits the valves of the reducing station to be opened wider to reduce erosion. When the fluid becomes hotter and more compressible, erosion is less severe and the setting for the valves can then be moved toa higher upstream pressure.

The high pressure heater stages drain, as shown, to the deareator storage tank 40 through line 84. The condensate from the condenser is pumped by the condensate pump 86 through the low pressure heaters 36 to the deaerator storage tank 40. At this stage, the main ow line valve 50, between the outlet end of the primary or platen superheater 22 and inlet end of the inishing superheater 24, is closed so that no flow passes through this valve. Also closed are the flash tank vapor discharge line valve 58 leading to the inlet end of the nishing superheater and the turbine by-pass linevalve 78, between the outlet end of the finishing superheater and the condenser 32. Also at this stage of start-up, the vapor lines from the ilash tank are closed by valves 62, 64, 66 and 82.

The burners are put into operation, for instance, at about 15% full load firing rate, controlled so that the furnace exit temperature entering the area of the tinishing superheater does not exceed 1200" F., the maximum safe temperature for the tube metal.

When the uid temperature upstream of the reducing station 44 'reaches a predetermined temperature', for instance about 300 F., the reducing valves of the station 44 are adjusted to increase the upstream pressure from 600 p.s.i. to 3650 p.s.i. (This would be a suitable full load pressure for the high pressure circuitry of a` superthis point, the ow remaining atr 30%, no signiiicant steam quantity is ashed in the flash tank, but as heating is continued, at constant pressure and iiow, steam as-h- Ving Will'take place in the flash tank in increasing quantities, and a level will be established in the ash tank.

This will occur prior to point A on FIG. 2.

Also prior to point A, the cycle water is cleaned up. This is effected through line 93 (shown as a dashed line `in FIG. l) leading from the outlet of the high pressure 'heaters to the condenser hot well. In conventional oncethrough units, the ilash tank drain llow is used for-this purpose. By this aspect of the invention, the temperature drop in the feedwater that would occur, if flash tank drain ow were used, is avoided.

At point A, the ii'ash tank pressure reaches its set point value of about 500 p.s.i., at which it is controlled until, in the start-up cycle, a predetermined turbine load, in this example, a 5% load, is achieved. 'I'he valve 58, in the Vapor line 56 leading from the ash tank vapor space to the inlet of the finishing superheater 24, is cracked open on remote manual control to furnish warming steam to the main steam line. Also, the valve 78 in the turbine by-pass line 76 is cracked open to allow the warming steam to pass to the condenser 32. As au alternative, any s-uitable main steam line drain may be provided for this purpose.

All of the flash tank drain flow is still routed through the valves 72 and 74 to the condenser hot well and high pressure heater. Subsequently, some steam is made available for the gland seal regulator and the deaerator. Valves 72 and 74 respond to level control (LC) to hold the desired flash tank liquid level.

Initial turbine rolling At point B on FIG. 2, about 5% of full load steam ow is available from the ash tank, and a portion is diverted through valve 58 and the finishing superheater for rolling and warming the high pressure turbine, the remainder being disposed of through the by-pass line 60 leading from the vapor space of the fiash tank to the high pressure heater 42, and elsewhere.

A typical high pressure steam turbine unit (FIG. 4) will include a main throttle stop valve 86, and a plurality of governor control valves 88 in series with the stop valve. Associated with the stop valve is a relatively small bypass control valve 90 for control of the ow at low loads.

The warming and rolling ow from the outlet of the ash tank, to the high pressure turbine through the finishing superheating section is superheated slightly in the finishing superheater to point G on FIG. 2. At this stage, the turbine throttle stop valve 86 (FIG.4) is closed and the by-pass valve 90 controls the flow to the turbine, reducing the .pressure of the'ow to about 50 p.s.i. V(point H, FIG. 2).

Usually, only about 2 to 3% flow is needed for the l purpose of warming and rolling the turbine, but if more than 2 to 3% ow is needed through the finishing superheater for control of the enthalpy of the uid at the turbine throttle, the remainder may be dumped through the turbine by-pass line 76, from the outlet of the finishing superheater, to the condenser. The spray attemperator 80, on temperature control, insures that the temperature of the fluid entering the condenser does not exceed design limits of the condenser.

During the final stages of heating of the generator with flash tank pressure held at 500 p.s.i. (points B to E, FIG. 2), and .prior to loading, the initial ring rate is maintained. Furnace exit temperature still is monitored and should not exceed 1200 F. and the 2 to 5% rollin-g flow for Warming the turbine is maintained from the ash tank to the finishing superheater and the high pressure turbine. A steady-state condition for the generator is achieved when the enthalpy is approximately 1050 to 1100 B.t.u./lb. entering the flash tank (point E), at which point the generator circuit components are up to temperature at the flow rate, firing rate, and pressure set in the unit. At this point, firing rate may be decreased to accommodate only turbine warming and cycle losses.

The dotted line between points C and D in FIG. 2 represents the pressure reduction in the main ow line from 3500 p.s.i. to 500 p.s.i. at the reducing station 44. The compressible supercritical nid at about 730 F. is expanded to a temperature of about 470 F. and a vapor content of about 75%. However, an advantage is obtained in that the transfer of heat to the reduced pressure fluid in the superheating sections is substantially greater.

For instance, for the same ue gas How over`the superheater bank `and fluid ow in the bank, and for the same thermodynamic entering conditions of flue gas and fluid, the pressure reduction from 3500 p.s.i. to 500 p.s.i. increases the bank heat transfer eiciency by 54% approximately.

When the enthalpy of the steam at the outlet of the primary section (22, FIG. 1) is slightly superheated (between points F and G), or is yat some other predetermined temperature, the main ow line valve 50 between the superheatin-g sections can be opened on remote manual so that the flow is from the primary superheater outlet directly to the finishing superheating section and to the high pressure turbine. The set point for the Hash tank still controls or regulates the pressure in the heating surfaces downstream of or following the reducing station 44.

Turbine .synchronization and loading As the heating is continued in the unit at constant pressure and flow, the steam temperature entering the turbine first stage through the turbine stop valve by-pass increases along the constant pressure line from point H to point K, concurrent with an increase in temperature of the throttle steam at 500 p.s.i. (temperature of the stem at the outlet of the nishing superheater) from point G to point J. At point I, the turbine parts reach a state of equilibrium, and the turbine is then sychronized and a load is applied and gradually increased to 5%, depend-l to 10% load (finishing superheater outlet conditions at' point J), is achieved by increasing thev flash tank set point from 500 p.s.i. to 1000 p.s.i. This is point S in FIGS. 5 and 6, the latter figure showing the relationship between load and governor volve position. At point I', the turbine is placed on governor valve control, byclosing down on the governor valves until they assume control, and the throttle stop valve (86, FIG. 4) is opened Wide. This is line S-S on FIG. 6. The load is increased to 30% by opening the governor valves to full open position along the 1000 p.s.i. throttle pressure line, J-L on FIG. 2, and S (or S)-T on FIGS. 5 and 6. The temperature and pressure of the fluid entering the turbine first stage follows approximately path J-L from 15% to 30% load. It will be recalled that a 30% full load start-up ow had been established through the unit for cooling the furnace circuitry. At 30% turbine load, the start-up system is removed from service (no longer being required for pressure control) and loading the turbine from 30 to 100% load is accomplished by combustion control regulation of throttle pressure and temperature in response to load demand. This is line L-M on FIG. 2 and lines T-V on FIGS. 5 and 6. The combustion control regulation includes means to control the feed pump output and to meter the fuel and air supply,

-so that the increase in throttle pressure and load with time is linear.

When the throttle steam pressure is approximately 3150 pLs.i. (point U, FIG. 5), the stop valve 94 around the pressure reducing station 44, in FIG. 1, is gradually opened on remote manual control for further increasing the throttle pressure, and the pressure reducing station, which is then at capacity, is taken out of service. At full throttle pressure, the valve 94 is fully open.

The advantages of the invention with respect to a cold start should now be apparent. Particularly evident should be the advantage of improved heat transfer in the superthrough a turbine throttle stop valve by-pass to the tur-V bine with governor valves Wide open. To design the oncethrough boiler furnace circuitry to operate at a reduced pressure condition requires extra expense in the design of the furnace circuitry. The introduction of throttling valves in the once-through boiler circuitry (for instance,

just downstream of the convection enclosure passes) en-V ables the unitf to operate with reduced pressures, at the turbine throttle and still permits the furnace circuitry to be designed to operate only at full pressure.

A further advantage is that the temperature difierential across the throttle stop valve is reduced. With 500 lbs. throttle pressure, throttle to'the 50 p.s.i required at the turbine irst stage inlet, the temperaure differenial across the turbine stop valve is only 100. If the throttle pressure were 3500 p.s.i., this dierential would be in the order of magnitude of 400 F.

Y Another advantage is that by having a stop valve 50 upstream of the turbine throttle stop valve, cold water is prevented from being in physical contact with the upstream side of the throttle stop valve. This is a condition which has long plagued operators of once-through units.

Full arc admission in the turbine unit is where all the turbine governor valves are in the wide-open position. Partial arc admission exists where one or more of the governor valves are open and some are closed. The use of a low pressure throttle steam and throttling through the turbine throttle stop valve by-pass permits the turbine governor valves to remain wide-open for full arc admission during the acceleration, synchronizing and initial loading period. By virtue of this, all the valve bowls are heated uniformly along with the valve chest and irst stage shell region.

The turbine is operated on governor valve control (this corresponds to partial arc admission) only during the intermediate loading period from 10% to 30% load. Operation at partial arc admission is thus limited to a very narrow Vload range which would, in normal practice, be passed through rapidly. From 30% to 100% load, the turbine is again loaded using f ull arc admission, by combustion control regulation of throttle steam in response to load demand to give required turbine load.

FIG. 7 illustrates, as an embodiment of the invention, a main ow line reducing station wherein a plurality of reducing valves are used. It isfound that Va fewer number of 'valves is required by usingV at least one double-port high capacity valve,.indicated by the numeral 102. This valve is not'a tight Yshut-off valve and requires an isolation shut-off valve 104 to be operated in conjunction with it. However, the saving is substantial. Whereas three reducing valves may handle the initial 30% minimum ow (items 44), ive additional valves could be required to handle the Vliow increase up to full ow and transfer to the main ow line valves 94. Instead, one double port valve can handle this additional ilow. In the arrangement illustrated, the reducing valves operate in sequence, with, in the case of increased dow, the double port valve 10-2 opening last. However, before the valve 102 can open, the shut-off valve 104 must open. Conversely, the shutoi valve 104 closes after the double port valve closes.

H ot re-start 'IVhe invention is also useful for hot re-starts. ReferringV to FIGS. 1, 3, and 3A, a hot re-start is initiated by estabcondenser hot well, and the ilash'tank steam How is routed to the gland seal regulator through valve 62, the deaerator through valve 64, the high pressure heater through valve V66, and to the condenser through line 60d and valve 82. The pressure reducing `station 44 ,maintains 3500 p.s.i.y fluid pressure in the furnace circuitry, and the flash tank set point is 500 p.s.i. The stop valves 94 and 50 in the main flow line upstream of and intermediate of the superheating sections are closed. Valve 58 in line 56 also is closed.

The burners are set at start-up iring rate (for instance 15% full load firing rate), and the main line valve S0 between the superheating sections is opened on manual control. At this moment, point C on FIG. 3 represents the state of the iluid entering the reducing station, and the location of this point or the enthalpy of the fluid entering the reducing station may very Ydepending on the period of Y shut-down of the generator. The line C-D represents the pressure reduction through the reducing station so that point D will correspondingly vary depending on the period of shut-down. Line D-E represents the enthalpy pickup in the primary and/ or platen superheater, and E-F-G the enthalpy pick-up in the inishing superheater. The point G can be arranged to be higher or lower along the 500 p.s.i. constant pressure line (i.e., the uid will have a higher or lower heat or enthalpy content) depending on the rate of flow through the finishing superheater and the amount of iiow diverted through the ash tank. Valve 58 between the iiash tank vapor space and the finishing superheater remains closed. Also during this initial firing period, the turbine by-pass valve 78 between the outlet of the linishingsuperheater and the condenser is opened to pass the dow through the finishing superheater to the condenser.

When the steam conditions at the outlet of the iinishing superheater, point G, match the temperature of the turbine inlet parts, point H (the steam conditions entering the turbine first stage), the turbine by-pass steam iiow is switched to the high pressure turbine by manually closing the by-pass valve 78 for initial rolling. The lpressure for point H is about 50 p.s.i. by virtue of the reduction in pressure through the throttle stop valve byfpass 90.k When the turbine is synchronized, a load is applied,Y

the change in steam conditions entering the turbine first stage when the load is applied. Following this, the load is Y increased to about 30% of full load by operation of the governor valves as with a cold start, closing the governor valves until control is assumed and then opening them to full open position at the throttle pressure of 1000 p.s.i. Y Y

This increase in first stage entering pressure and corresponding mcrease in turbine load is represented by line .I-K on FIGS. 3 and 3A. At point K, the stop valve 52- in the 4by-pass line leading to the flash tank is closed by remote manual means removing the start-up -by-pass sys. tem from service. From this point on, aspects of the cold start are repeated to full load, -point M. v

' The simplicity and suitability of the system for a hot re-start should be evident.

.In providing means for reheatring the throttle steam prior to admission to the turbine, an operator is enabled to better match irst stage inlet steam temperature withv the temperature of tur-bine inlet parts (point H). If full throttling is -undertaken through the turbine throttle valve to point H, the temperature of the inc-oming'uid would fall below that of the turbine inlet parts, or at least it would make it diicult to match the temperature with.

turbine inlet parts. By obtaining the exact enthalpy Vrequired at the outlet of the finishing superheater in a iiuid already throttled to a low pressure, the temperature of the liuid can be reduced to that. of the turbine parts by the turbine throttle valve lay-pass.

In addition, it is evident from FIG. 3 that the start-up can easily and efficiently be effected from any heat level or enthalpy level of the circulating fluid. For a quick start, a minimum degree of manual control is required, and the load is smoothly applied to the turbine for maximum protection of turbine parts. Other advantages of the cold start are also realized.

For instance, the invention has the same -advantage as that in the initial start-up period in that all of the valve bowls are heated evenly, all being wide open during most of the start-up period. This enables a closer approach to ideal starting and loading of the turbine than heretofore achieved, and from this, a reduced turbine maintenance cost.

Low load The invention is also useful for low load control, which may be affected either by Variable pressure and temperature operation at the turbine throttle, or by turbine governor Valve control at constant pressure and temperature.

In connection with the latter, a minimum flow, for instance a 30% flow, is required in the high pressure furnace circuitry for cooling the circuitry. In reducing the turbine load, through turbine governor valve control of the ow no problem is experienced as the flow is decreased to this minimum flow by combustion control regulation of the feed pump and firing rate. The ow is in the main flow path valve 94, FIG. 1, by-passing the pressure reducing station 44, so that full throttle pressure is maintained at the turbine inlet.

Below 30% flow to the turbine, the start-up system is advantageously used to dispose of the excess flow at the primary superheater outlet. However, for this purpose, the pressure reducing valve 96 in lone 98, by-passing the stop valve 52 between the primary superheater outlet and the flash tank, handles the flow, holding full pressure at the turbine throttle. The pressure set point for the reducing valve 96 will not respond at 30% load and below, as indicated by the turbine first stage outlet pressure load signal, represented by dotted lines leading from the turbine to control box 110 for valve 96. If the capacity of the reducing valve 96 is exceeded, i.e., more flow must be diverted than the valve is capable `of handling, the additional flow is passed through valve 78 to the condenser. This valve also is under set point regulation in response to the pressure (pressure controller 112) at the finishing superheater outlet. Subsequent manual adjustment of by-pass valve 52 may also be employed to increase the iow to the start-up by-pass system.

At very low loads, or on sudden changes in load, the fluid temperature at the primary supenheater outlet may be too high, and can be reduced by means of spray attemperators 114 in response to an 1anticipatory turbine load signal (first stage turbine outlet pressure) and to the main steam temperature set point as compared to the measured temperature.

f The role of variable pressure and temperature operation in the system is somewhat different. If prolonged operation lat loads below the minimum required, for instance 30%, is desired with the start-up by-pass system in service, the pressure reducing station 44 is adjusted to hold upstream set pressure required for the supercritical or subcritical unit (for instance 3650-p.s.i. or 2550 p.s.i. respectively), and the by-pass Valve 94 is slowly closed on remote manual control. When the pressure downstream of the reducing station approaches flash tank pressure, the stop valve 52 is fully opened on remote manual control 'and the turbine governor valve positioner will open to hold constant load at the lower throttle pressure.

The controls Controls for the generator-turbine unit have been mentioned in the specification in greater or lesser detail.

In the cold start-up sequence, the feed pump (34, FIG. 1) is on ow control to hold the minimum flow (for instance, 30% of full load flow). This -is iu response to a signal from flow orifice 122 acting through controller 123 and signal 124. The pressure reducing valve 44 is on pressure control (signal 125) to hold the set point pressure upstream of the valve. The flash tank subtemperature for the high pressure turbine (point G, FIG.-

2.) is achieved by manual adjustment of the firing rate and control of the flow through the finishing superheater (excess flow passing through the throttle stop valve bypass valve 78). It will be recalled that the throttle stop valve (90, FIG. 4) admits aV 2-3% flow to the turbine first stage. Manual control of the turbine by-pass valve (78, FIG. l) passes a greater or lesser flow through the finishing superheater for control of the temperature at the finishing superheater outlet.

Up to loading the turbine (point J, FIG. 2), the temperature of the fluid entering the turbine is .gradually increased Iat constant ow to the lturbine and constant pressure (500 p.s.i.) from points G to I, the fuel and air input to the burners being subject to minor manual adjustment from the initially set firing rate. As previously indicated, after asteady-state condition for the generator is achieved (point E), the addition-al heat supplied is used for turbine warming and cycle losses.

At point I, the turbine parts reach a state of equilibrium, and the turbine is synchronized and loaded up to, for example, 5% of full load. This is accomplished by closing the turbine by-pass valve A78, and the 4full low through the finishing superheater, for instance a 5% flow, is fed to the turbine. Since the by-pass control (valve 7-8) is no longer used for temperature control, the temperature at the finishing superheater outlet is controlled and held at a set point (point J, FIG. 2) by means of a temperature controller (130, FIG. 1). To the controller is fed a temperature signal 132, the signal 142 from the temperature controller 130, compared to anticipatory load signal 139, being transmitted by signal 133 to the emergency spray attemperators 114 located immediately upstream of the finishing superheater. It will be recalled that of the 30% start-up flow, about 5% only, at this stage, is passed through the finishing superheater to `the turbine, the remainder being disposed of in the flash tank by-pass system.

During this period of loading to 5%, it is envisioned that although the fuel tiring rate and air input responds primarily to manual control 128, it will adjust for feed and main steam temperature to `a limited extent. For instance, a main steam tempera-ture 132 below the set point will increase the firing rate (signal 142), whereas above the set point, with full attemperation, it will decrease the firing rate. The feed Water temperature (signal 144b) falso is compared to the main steam temperature (signal 142). This combined signal then modifies the manual control signal (12S-129) for control of the fuel and -air supply. The purpose of the feed water temperature signal is, that when the `by-pass system is used, with flow to the high pressure heaters (for conserving heat), the feed water takes an initial jump in temperature. This must be compensated for by reducing the fuel and air input.

At la 5% load and above, control of the fuel and air input can be changed from manual to automatic by providing, for automatic control, a load demand signal 138, for instance a 5% demand signal which represents the load applied to the turbine. This signal is compared with Y required for to 30% load, the temperature compensated load signal 139 controls the firing rate.

. pressure `control 11 turbine iirst stage outlet pressure 136 and throttle pressure 137 to provide la load signal 139. The load signal 139 is then modified in a true control sense with the main steam temperature signal 142 and feed temperature signal 14412 as described above, the resultant signal controlling Ithe fuel and air input.

To achieve -a load increase above the as-h tank pressure set point Imay be immediately adjusted .to 1000 p.s.i. from 500 p.s.i., increasing the load to Control is then shifted from the throttle valve to the governor valve, and the load is increased at -constant pressure to 30% (point L) by moving the vgovernor valve position to -full open This increases the ilow to the turbine fromV 10% to 30%, Aand at 30% ow the flash tank start-up system can be removed from service, being no longer handling an excess flow. In this range 10% Following removal of the start-up system from service, the feed pump 34 responds also to the load demand signal 139b, and the heat input, flow rate, and pressure increase, preferably along the constant enthalpy line LM of FIG. 2, programmed in Ia suitable manner with the load signal 139 and temperature signal 142. As indicated above, the pressure reducing valve (or valves) 44 under 125 opens Iwider to accommodate the increased flow while maintaining the desired upstream pressure.

, For low load operation of the turbine, at constant throttle pressure, the iiow is initially reduced (down to the minimumV 30% fiow) by combustion control regulation of pumping and firing rate (load demand signal 138 to the feed pump 34 and to metered fuel and air system 126). At and below 30% ow, the turbine first stage load measuring signal 136 fed to control 110 for valve 96, will permit this valve to respond .to set point pressure control 145. Other aspects of low load control should be apparent from the earlier description on low load operation, and the above description covering control of the fuel and air input or firing rate.

It should be'noted that the reason increasing the -ash tank pressure from 500 to 1000 p.s.i. stems from a desire to obtain maximum load within the capacity of the throttle stop valve by-pass 90, FIG. 4, this capacity being reached at 5% load, for. 500 p.s.i. throttle pressure, or 10% load for 1000 p.s.i. throttle pressure. Governor val-ve control could be instituted at 5% load, but the valves would then have to assume controlV at a open position instead of 30% open position.

Although the invention has been described with respect to specific embodiments, many other variations within the spiritand scope in the invention as defined in the following claims will be apparent to those skilled in the art. Y

What is claimed is:

during start-up for 1. A Vmethod for starting-up of low load operation ofV a once-through vapor generator-turbine unit which includes the steps of establishing a minimum ow at operating pressure in the high temperature circuitry of the generator for cooling the circuitry; reducing the pressure of the flow upstream of convection heating surfaces of the generator whereby the flow is reheated at a reduced pressure; passing a portion of said reduced pressure flow to the turbine to satisfy load requirements on the turbine and passing the remainder of the ow at least in part in heat exchange with the feed to the generator; increasing or decreasing the ow to the turbine in response to load requirements;.providing 'said generator with a signal which in a rst instance represents a manual set point forload and in a second instance represents a required load demand as compared to turbine irst stage pressure and turbine throttle pressure; modifying said signal to a limited extent in the first instance and in a'true control sense in the second instance with aY second signal which is a function of the feed temperature and the temperature of the ow at the turbine throttle; and causing said modified signal to control the heat input to the generator.

2. A method for starting-up a once-through vapor generator-turbine unit comprising the steps of establishing in the high temperature circuitry in the generator a ow rate at operating pressure for the generator equal to the minimum ow required for cooling the circuitry; reducing the pressure of the ow upstream of convection heating surfaces of the generator whereby the ow is reheated at a reduced pressure; in one period of start-up passing ,aV

portion of said reduced pressure'flow to the turbine to satisfy initial load requirements on the turbine and passing the remainder of the flow at least in part in heat ex.- change with the feed to the generator; providing said generator with a signal which represents a manualset point for load, and modifying said signal to a limited extent with a second signal which is a function of the feed ternperature and the temperature of the flow at the turbine ,Y

throttle; in a next period of start-up increasing the flow to the turbine at a constant pressure in response to an increase in load until said minimum flow in full is fed to the turbine, providing said generator with an automatic signal which represents load demand as compared to turbine iirst stage pressure and turbine throttle pressure, and modifying said automatic signal in a true control sense with said second signal; and causing said modified signals to control the heat input to the generator, said second signal compensating for the reduction in heat transfer to the feed resulting from the increase in Viiow to the tux'- bine.

3. A method for starting up a once-through vapor gencrater-turbine unit comprising the steps of establishing in the high temperature circuitry in the generator a ow rate at operating pressure equal to the minimum ow ref quired for cooling the circuitry; reducing the pressure of the flow upstream of convection heating surfaces ofthe generator whereby the flow is reheated at a reduced pressure; in one period of start-up passing a portion of said reduced pressure flow to the turbine for initial loading of the turbine and passing the remainder of the ow at least in part in heat exchange with the feed to the generator; in a next period of start-up increasing the flow to the turbine in response to an increase in load until said minimum ow in full is fed to the turbine, providing saidgenerator with a load demand signal to control the Y the temperature of the fluid at the turbine throttle in said one period by controlling the ow in final heating surfaces of the generator and by-passing excess ow around the turbine and in said next period by attemperating the uid with the introduction of a cooler fluid therein.

4.1In a method for controlling a once-through vapor generator-turbine unit wherein during startfup and at low loads a portion of the heated uid ow may be used to preheat the feed flow, the steps comprising comparing a load demand signal with the turbine rst stage outlet pres- Y sure and the turbine throttle pressure to produce a load signal, using said load signal to control the fuel and-air input to the burners, and modifying said load signal rela-` tive to the turbine throttle steam temperature and the temperature of said feed flow to adjust the fuel and air input compensating for a change in the temperatures of the feed ow.

5. In a method in accordance with clainr 4 further including the steps of adjusting the pumping rate for said feed flow by means of a ow rate signal, correcting said flow rate signal relative to the temperature of thefeed ow, and comparing said temperature corrected liow rate signal with said load signal.

6. In a method in accordance with claim 5 wherein 13 said ow rate signal is provided with a minimum iiow setting to prevent the feed flow from decreasing below a minimum value.

7. A control system for a once-through vapor generator-turbine unit which includes means for diverting a portion of the heated uid ow to preheat the feed flow at low loads and start-up, comprising means for comparing a load demand signal with the turbine first stage outlet pressure and the turbine throttle pressure to produce a load signal, means for using said load signal to control the fuel and air input to the unit, and means for modifying said load signal relative to the turbine throttle steam temperature and the temperature of the feed flow to adjust the fuel and air input compensating for a change in temperature of the feed ow.

8. A control system in accordance with claim 7 further including means immediately upstream of the vapor gencrater-turbine unit for detecting the rate of flow of feed thereto and providing a flow signal proportional to the rate of ow, means responsive to said flow signal for adjusting the pumping rate for said feed iow, means for correcting said flow signal relative to the temperature of the feed flow, and means for comparing said temperature corrected ow signal with said load signal.

9. A control system in accordance with claim 8 wherein said means for adjusting the pumping rate for the feed ow is provided with a minimum flow setting to prevent the feed ow from decreasing below a minimum value.

10. A control system in accordance with claim 7 further including spray attemperator means arranged to introduce a cooling iiuid into the heated iiuid How, and means to control said spray attemperator means to maintain a set-point uid temperature at the turbine throttle.

11. A control system in accordance with claim 7 wherein said means for diverting a portion of the heated fluid ow includes means for reducing the pressure of the diverted uid, further including control means by which said pressure reducing means maintains a set-point uid pressure at the turbine throttle.

No references cited. MARTIN P. SCHWADRON, Primary Examiner. R. R. BUNEVICH, Assistant Examiner. 

1. A METHOD FOR STARTING-UP OF LOW LOAD OPERATION OF A ONCE-THROUGH VAPOR GENERATOR-TURBINE UNIT WHICH INCLUDES THE STEPS OF ESTABLISHING A MINIMUM FLOW AT OPERATING PRESSURE IN THE HIGH TEMPERATURE CIRCUITRY OF THE GENERATOR FOR COOLING THE CIRCUITRY; REDUCING THE PRESSURE OF THE FLOW UPSTREAM OF CONVECTION HEATING SURFACES OF THE GENERATOR WHEREBY THE FLOW IS REHEATED AT A REDUCED PRESSURE; PASSING A PORTION OF SAID REDUCED PRESSURE FLOW TO THE TURBINE TO SATISFY LOAD REQUIREMENTS ON THE TURBINE AND PASSING THE REMAINDER OF THE FLOW AT LEAST IN PART IN HEAT EXCHANGE WITH THE FEED TO THE GENERATOR; INCREASING OR DECREASING THE FLOW OF THE TURBINE IN RESPONSE TO LOAD REQUIREMENTS; PROVIDING SAID GENERATOR WITH A SIGNAL WHICH IN A FIRST INSTANCE REPRESENTS A MANUAL SET POINT FOR LOAD AND IN A SECOND INSTANCE REPRESENTS A REQUIRED LOAD DEMAND AS COMPARED TO TURBINE FIRST STAGE PRESSURE AND TURBINE THROTTLE PRESSURE; MODIFYING SAID SIGNAL TO A LIMITED 